Oil country tubular goods such as tubing and casing used for exploitation of crude oil or gas oil are connected with each other by threaded joints for pipes. A typical threaded joint for pipes used for connecting oil country tubular goods has a pin-box structure. A pin is a joint component having male (external) threads, and a box is a joint component having female (internal) threads. Typically, a pin is formed on the outer peripheral surface of both ends of a steel pipe used as an oil country tubular good, and a box is formed on the inner peripheral surface of both sides of a coupling, which is a separate member. In special threaded joints having excellent gas tightness, an unthreaded metal contact portion having a seal surface and a shoulder surface (also referred to as a torque shoulder) is formed at the end of the male threads of a pin and at the base of the female threads of a box. The threaded portions and the unthreaded metal contact portions form the contact surface of a threaded joint for pipes. With this type of threaded joint for pipes, excellent gas tightness is obtained by inserting one end of an oil country tubular good into a coupling and threadingly engaging the male threads and the female threads until the unthreaded metal contact portions of the two members contact each other to form metal-to-metal seals.
FIG. 6 is an explanatory view schematically showing the assembled structure of a typical threaded joint for pipes and showing the state of a steel pipe for an oil country tubular good and a coupling at the time of shipment. As shown in this figure, a pin 1 having a male threaded portion 3a is formed on the outer peripheral surface of both ends of a steel pipe A, and a box 2 having a female threaded portion 3b is formed on the inner periphery of both sides of a coupling B. The coupling B is previously connected to one end of the steel pipe A. As shown in FIG. 6, a coupling-type threaded joint for pipes is normally shipped in a state in which the coupling B is connected to the steel pipe A.
Accordingly, at the time of shipment, of the two pins and two boxes, one pin and one box are connected to each other. Although not shown in this figure, on the other pin and box which are not used for connection to each other, namely, the left-hand pin and the right-hand box in the figure, a pin protector and a box protector, respectively, are mounted prior to shipment in order to protect the contact surfaces of the pin and box against rusting, scratches, and infiltration of foreign matter. These protectors are removed before use.
A pin protector and a box protector each typically comprise a tubular body which is somewhat longer than the threaded joint and which is normally made of a resin or a metal-reinforced resin. These protectors are typically closed at one end or in the vicinity of one end, but there are also protectors which are open at both ends.
A pin protector has female threads on its inner peripheral surface which threadingly engage with the male threads of a pin, and it is mounted on a pin by thread engagement. Similarly, a box protector has male threads on its outer peripheral surface which threadingly engage with the female threads of a box, and it is mounted on a box by thread engagement. At this time, the pin protector is sufficiently tightened until its female threads interfere with the male threads of the pin so that the protector will not fall off even if it receives an impact during transport or handling after shipment. Similarly, the box protector is sufficiently tightened until its male threads interfere with the female threads of the box. A pin protector and a box protector including their threads are normally manufactured by injection molding, and the threaded portions of a pin protector and a box protector are formed of the same resin as the main body of the protectors.
In order to guarantee galling resistance and gas tightness at the time of makeup of oil country tubular goods, a viscous liquid lubricant referred to as a compound grease or dope and containing a heavy metal powder has been applied in the field to the contact surfaces (the threaded portions and the unthreaded metal contact portions) of threaded joints. Such a compound grease is prescribed by API BUL 5A2. Compound grease also has a corrosion-preventing function which prevents rusting of the contact surface to which it is applied.
In the past, when mounting a pin protector or a box protector on a pin or a box, compound grease or other lubricating grease (such as a lubricant referred to as green dope which does not contain a heavy metal powder) has been applied to the contact surface. The lubricating grease has fluidity and can fill the space between a protector and a pin or a box. Therefore, even if a protector is not specially provided with a seal mechanism, the contact surfaces of the pin and box are isolated from the exterior, and a corrosion-preventing function and a function of preventing infiltration of foreign matter are achieved.
However, in recent years, environmental regulations are becoming stricter around the world, and there is a demand for threaded joints for pipes which can be made up without using compound grease since compound grease contains a large amount of heavy metal powder which can have an adverse effect on humans and living organisms. It is also desired that lubricating treatment of threaded joints for pipes in the field be made unnecessary in order to increase the operating efficiency.
A typical example of such a threaded joint for pipes is one like that disclosed in Patent Document 1, for example, in which the contact surface of one or both of a pin and a box is coated with a solid lubricating coating having a lubricating powder (such as molybdenum disulfide or graphite) dispersed in a resin.
Patent Document 2 discloses a threaded joint for pipes in which the contact surface of at least one of a pin and a box is coated with a two-layer coating having a viscous liquid or semisolid lubricating coating and atop it a dry solid coating. The dry solid coating may be a coating of a thermosetting resin such as an acrylic resin or a coating of an ultraviolet curing resin.
Patent Document 3 discloses a threaded joint for pipes in which a thin, non-tacky lubricating coating comprising a lubricating powder dispersed in a solid matrix exhibiting plastic or viscoplastic rheological behavior (flow properties) is formed on the surfaces of the threads of a pin and a box. The matrix preferably has a melting point in the range of 80-320° C., and the coating is formed by hot melt spraying, flame coating using a powder, or spray coating of an aqueous emulsion.
Patent Document 4 discloses a threaded joint for pipes in which the contact surface of at least one of a pin and a box is coated with a two-layer coating having a solid lubricating coating comprising a lubricating powder and a binder, and atop it a solid corrosion-preventing coating which does not contain solid particles.
Similarly, a protector which is mounted on a threaded joint for pipes having such a lubricating coating and particularly a solid lubricating coating formed thereon and which is made up without using a compound grease is preferably mounted on a pin or a box without using a lubricating grease. However, in this case, a seal is not provided by a lubricating grease, so it is necessary for a pin protector or a box protector itself to have sealing properties. There have been some proposals in this respect in the prior art.
For example, as shown in FIG. 7, Patent Document 5 discloses a box protector 4 for a threaded joint for pipes having sealing properties in which the protector has a first seal portion 4b and a second seal portion 4c each in the form of an elastic annular projecting body made of the same material as the protector body 4a in the positions opposing the shoulder stoppers 5a and 5b of a box 5.
As shown in FIG. 8, Patent Document 6 discloses a box protector 6 for a threaded joint for pipes having a seal of increased reliability by using an elastic seal ring 6c which is mounted on the outer peripheral surface 6b of the protector body 6a opposing the end surface 7a of a box 7 as a first seal portion and using the end surface 6d opposing the torque shoulder 7b of the box 7 as a second seal portion.
Patent Document 1: JP 09-72467 A1
Patent Document 2: WO 2006/104251
Patent Document 3: WO 2007/042231
Patent Document 4: WO 2006/75774
Patent Document 5: JP 2003-240188 A1
Patent Document 6: WO 2011/027433